LANDSLIDES and LINEAR INFRASTRUCTURE in WEST-CENTRAL BRITISH COLUMBIA Marten Geertsema BC Forest Service, Prince George, BC Canada [email protected] James W
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LANDSLIDES AND LINEAR INFRASTRUCTURE IN WEST-CENTRAL BRITISH COLUMBIA Marten Geertsema BC Forest Service, Prince George, BC Canada [email protected] James W. Schwab BC Forest Service, Smithers, BC Canada Andrée Blais-Stevens Geological Survey of Canada, Ottawa RÉSUMÉ Les mouvements de terrain destructifs sont communs à l’ouest central de la Colombie-Britannique (C.-B.). Ces mouvements incluent des coulées et des glissements de débris, des coulées de terre et d’argiles, des chutes de blocs, des glissements et des avalanches rocheuses et des mouvements complexes impliquant la roche et le sol. Les pipelines, les lignes de transmission électrique, les routes et les chemins de fer ont tous été affectés par ces types de mouvements, perturbant le service aux communautés. Nous fournissons des exemples de mouvements destructifs, de leurs impacts, et des conditions climatiques liées aux glissements de terrain. Nous considérons également les aléas de glissements de terrain pour l'ouest central de la C.-B. sous certains scénarios du changement climatique. ABSTRACT Destructive landslides are common in west central British Columbia (BC). Landslides include debris flows and slides, earth flows and flowslides, rock falls, slides, and avalanches, and complex landslides involving both rock and soil. Pipelines, hydro transmission lines, roads and railways have all been impacted by these landslides, disrupting service to communities. We provide examples of the destructive landslides, their impacts, and the climatic conditions associated with the failures. We also consider future landsliding potential for west central BC under climate change scenarios. Figure 1. Landslides and linear infrastructure in west central BC. The landslides shown are described in the text and represent a small portion of actual landslides in the area. In : J. Locat, D. Perret, D. Turmel, D. Demers et S. Leroueil, (2008). Comptes rendus de la 4e Conférence canadienne sur les géorisques: des causes à la gestion. Proceedings of the 4th Canadian Conference on Geohazards : From Causes to Management. Presse de l’Université Laval, Québec, 594 p. M. Geertsema et al. 1. INTRODUCTION zones. At higher elevations, the Mountain-Hemlock, Engelmann Spruce-Subalpine Fir and Alpine Tundra zones West central British Columbia (BC) is a rugged, but sparsely occur ( Meidinger and Pojar 1991). populated portion of the province (Figure 1). Nevertheless the area hosts important transportation corridors, linking its 2.3 Bedrock geology communities and providing Canadian access to the Orient. Infrastructure includes roads, railways, hydro transmission On the coast, the bedrock lies within the Coast-Cascades lines, and hydrocarbon pipelines. The cities of Kitimat and Belt and belongs to the Coast Plutonic Complex. It is mainly Prince Rupert are important deep sea ports that are composed of Paleozoic to early Tertiary granitic rocks and expected to undergo substantial growth to meet the needs Proterozoic to Paleozoic high-grade metamorphic rocks. of growing Asian economies. Recently, major oil pipelines Towards the east, the bedrock is part of the Intermontane have been proposed, connecting the North American Belt, which consists of sedimentary and volcanic rocks of network to a Pacific tidewater port at Kitimat. Jurassic-Cretaceous age intruded by Cretaceous-early Tertiary felsitic to porphyritic plugs (Duffel and Souther The purpose of this paper is to provide examples of various 1964; Clague, 1984). types of destructive landslides that impact linear infrastructure in west central BC. The list is by no means 2.4 Surficial geology all-inclusive, and many more events have occurred that are not recorded here. We also consider the impact of The surficial geology of the area was mapped and described projected climate change on the incidence of future by Clague (1984). During the last glaciation, approximately landslides. 15 ka BP, the area was covered by the Cordilleran Ice Sheet. The types of sediments deposited were mainly cobbly to bouldery tills. During glaciation, the weight of the 2. SETTING ice sheet caused isostatic depression of the Earth’s crust. During deglaciation, (11-12ka BP), the crust slowly 2.1 Physiography rebounded, but at a much slower rate than the melting of the ice sheet. This caused coastal river valleys to be inundated The study area is located within the Coast, Hazelton and by marine waters. Glaciomarine sediments, mainly Skeena mountain ranges of northwestern British Columbia laminated silts and clays, were deposited as glaciers melted (Holland, 1976). These mountains are part of the northwest and retreated up the valleys. In areas where ice was at a trending geological zones within the Canadian Cordillera standstill for an extended period of time (e.g., Terrace), called the Coast-Cascades Belt and the Intermontane Belt. deltaic sands and gravels were deposited. The area consists of rugged mountains cut by deep valleys filled with glacial and post-glacial sediments. Most A dynamic interpretation of the kinematics of rock slides was mountains have rounded, dome-like tops from glacial not offered until 1932 (Heim, 1932, pp 126-136). Skermer’s overriding with relatively uniform elevations varying between translation noted “Since it is a question of finding the 1800-2400 m. Higher peaks, up to 2800 m, project as extreme limits of the imminent landslide, the energy line has nunataks (Holland, 1976; Clague, 1984). to be drawn down from the highest point of the probable slide mass” (p. 130) However, “ the inclination of the straight 2.2 Climate and vegetation line between the uppermost scarp edge and the tip of the tongue of the stationary rubble stream has long since been Topography strongly influences air temperature and considered a characteristic factor for landslides”( p. 129) precipitation in the area. Temperatures decrease with and ‘After several studies, I found the character of a rock increasing elevation and distance inland. For example, in avalanche could best be compared by using the angle of January, the mean coastal temperature at Prince Rupert (52 reach of the central stream line.” (p. 106). For the purposes masl), is about 1.8ºC. In comparison, roughly 350 km in- of comparison, we, too, have used this angle, termed “the land, in the Bulkley Valley at Smithers (515 masl), the mean travel angle” by Cruden and Varnes (1996, Figure 3-7) in temperature is about -10.5ºC. Moreover, mean July this paper. We remember that, in some cases, it is a temperatures vary between 14-17 ºC in the valleys and conservative estimate of the peak travel angle first decrease by several degrees in elevation (Clague, 1984). employed in 1911. The travel angle of the Frank Slide along profile 9 is 15.4°. The predominant flow of moisture–laden air travels towards the east from the Pacific Ocean. Mean annual precipitation values for coastal mountains exceed 2500 mm and can be 3. LANDSLIDE TYPES IMPACTING LINEAR greater than 3500 mm in some areas. There is a marked INFRASTRUCTURE decrease in precipitation towards the east. For example, in the Bulkley Valley, mean annual precipitation values range Numerous types of landslides are common in the varied between 400-500 mm (Clague, 1984). terrain of west central BC. We provide examples of a few of the landslides that have damaged linear infrastructure in the The main valleys of the study area lie within the Coastal region, but also make reference to a number of significant Western Hemlock and Sub-Boreal Spruce biogeoclimatic landslides that have not damaged infrastructure. Included Landslide and linear infrastructure in West-Central British Columbia are large rock slides, some of which transformed into debris avalanches and/or flows, debris flows, debris slides, earth flows in glacial lake sediments and in fine textured colluvium, and flowslides in sensitive glaciomarine sediments. The locations of the described landslides are shown in Figure 1. The landslides damage infrastructure in two main ways. (1) Most commonly infrastructure is in the landslide runout zone and is struck by moving debris. (2) Occasionally infrastructure is placed on unstable ground and is moved suddenly or episodically as the main body (Cruden and Varnes 1996) of the landslide moves. Where rock slides, debris flows, and debris slides described here resulted in damage to infrastructure, it was always the result of the first case, where objects were struck by moving material. In contrast, earth flows and flowslides damaged infrastructure by carrying it along during movement. Landslides also can have varying causes and triggers. Examples of causes can be inherent weakness in material, valley deepening, glacial debuttressing, road construction, excavation etc. Triggers can be rainfall, snowmelt, site loading, seismic activity, etc. Shallow landslides such as debris slides and flows are often triggered by intense rainfall. Deep seated landslides such as large rock slides, earth flows and flowslides tend to have longer hydrological response times (Egginton 2005). 3.1 Rock slides Figure 2. Large, long runout rock slides in west central BC. Six large rock slides have occurred in west central BC since See Figure 1 for locations. (A) The 1999 Howson rock slide 1978, five of which occurred since 1999, and four since entrained till and colluvium, severing a natural gas pipeline 2002. Three of the six rock slides severed a natural gas (1). Note the hydro transmission line (2) and the landslide pipeline. All of the rock slides had relatively long runouts. dammed lake (3). (B) The 2002 Zymoetz rock slide transformed into a channelized debris flow, dammed the The 1999 Howson rock slide (Figure 2a) occurred at 03:00 Zymoetz River, flooded an important Forest Service Road hours PDT on 11 September. The rock slide travelled 2.7 for more than a year, and ruptured a pipeline. The landslide km, entraining till and colluvium. It ruptured the natural gas traveled 4 km. (C) The 2002 Harold Price rock slide pipeline, and formed a lake. The initial rockslide involved 0.9 transformed into a debris avalanche and then into a M m3 and the entrained volume is estimated to be 1.5 M m3 channelized debris flow, traveling more than 4 km.